N-vinyl-2-pyrrolidone (hereinafter, abbreviated as “NVP”) is useful as an reactive diluent, and polymers of NVP are used extensively in various fields such as pharmaceuticals, cosmetics, adhesives, paints, dispersants, inks, electronic parts and photoresist materials because of their advantages such as biocompatibility, stability and hydrophilicity.
Impurities are firmly required to be removed in these fields. Therefore, it is proposed to control the concentration of impurities in a mother liquor at a constant level in search of a method for obtaining high-purity NVP (JP-A-2004-345994).
A method for obtaining NVP of extremely high purity while improving energy efficiency by reducing stages in a crystallization process is proposed (JP-A-2004-345993).